1) Field of the Invention
This invention relates to a magnetic fluid sealing device and its manufacturing process, and more specifically to a magnetic fluid sealing device suitable for use in ensuring the maintenance of airtight sealing, for example, at the position where a drive shaft for a hard disk assembled in a computer extends through a casing and also to its manufacturing process.
2) Description of the Related Art
A hard disk assembled in a computer or the like has a disk provided as a magnetic recording element inside a casing. This disk is driven by a motor provided outside the casing. Since the hard disk itself extremely disfavors fouling by dust and the like, a sealing device must be provided at the position where the drive shaft of the motor extends through the casing so that dust penetration can be prevented.
As sealing devices suitable for provision at such a position to prevent penetration of dust and the like, magnetic fluid sealing devices have been employed commonly, including those disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. HEI 62-110080, Japanese Utility Model Application Laid-Open (Kokai) Nos. HEI 58-191423, HEI 61-13025, HEI 61-44067, HEI 61-79070, HEI 62-195261, HEI 61-204027, HEI 63-8419 and HEI 63-139325, Japanese Utility Model Publication (Kokoku) No. SHO 63-29944, and U.S. Pat. Nos. 4,628,384 and 4,692,826.
The magnetic fluid sealing devices disclosed in the above prior art publications basically have a structure similar to that illustrated in FIG. 18 although their structures are different in various ways in details.
In FIG. 18, numeral 1 indicates a housing made of a non-magnetic material such as aluminum or a synthetic resin. The housing 1 is secured, for example, on the casing described above. Designated at numeral 2 is a shaft made of a magnetic material such as iron. A main body 3 of a magnetic fluid sealing device is arranged within a cylindrical space defined between an inner circumferential wall 4 of the housing 1 and an outer circumferential wall 5 of the shaft 2.
In the main body 3 of the above magnetic fluid sealing device, an annular permanent magnet 7 magnetized in an axial direction, i.e., in the horizontal direction as viewed in FIG. 18 is held between a pair of pole pieces 8,9 formed in an annular shape with a magnetic material. A magnetic fluid 10 is retained by the magnetic force of the permanent magnet 7 in gaps 11,11 formed between inner circumferential edges of the pole pieces 8,9 and the outer circumferential wall 5 of the shaft 2, respectively. The permanent magnet 7 and the paired pole pieces 8,9 have an outer diameter either equal to or slightly greater than the inner diameter of the housing 1, whereby the main body 3 of the magnetic fluid sealing device, said main body being constructed of the members 7,8,9,10, is fitted in and fixed on the inner circumferential wall of the housing 1.
The main body 3 of the magnetic fluid sealing device is constructed between the inner circumferential wall 4 of the housing 1 and the inner circumferential wall 5 of the shaft 2 as described above, the sealing between the inner circumferential wall 4 of the housing 1 and the outer circumferential wall 5 of the shaft 2 can be maintained by the magnetic fluid 10 held between the outer circumferential wall 5 of the shaft 2 and the inner circumferential edges of the respective pole pieces 8,9 despite rotation of the shaft 2 inside the housing 1.
Incidentally, it is not absolutely necessary to use the pole pieces 8,9 in combination. A magnetic fluid sealing device can be constructed with a single pole piece provided that a magnetic field can be formed by a bearing or the like provided in the vicinity of the main body 3 of the magnetic fluid sealing device.
In a magnetic fluid sealing device constructed and used as described above, the magnetic fluid 10 formed by dispersing fine particles of a ferromagnetic material in an oil having extremely low volatility such as a hydrocarbon mineral oil, synthetic oil, ester oil, ether oil or silicone oil tends to spread over a wide area on the surface of the metal-made shaft 2 because of the strong wettability of the fluid for the surface of the shaft 2.
If the magnetic fluid 10 spreads over a wide area on the surface of the shaft 2 as described above, the surface area of the magnetic fluid 10 becomes greater. As a result, the evaporation loss of the dispersant oil increases (in a long period even if the oil has low volatility) and the service life of the magnetic fluid sealing device is therefore shortened due to the decreased amount of the magnetic fluid 10.
If the magnetic fluid 10 spreads along the surface of the shaft (or along the side walls of the pole pieces 8,9 and the inner circumferential wall of the housing 1 in the case of a different structure), the risk that the magnetic fluid 10 may scatter in a clean space with a hard disk provided therein becomes higher.
If the magnetic fluid 10 scatters in the clean space and sticks on the hard disk or the like accommodated inside the clean space, the magnetic fluid 10 may become a cause for a malfunction of a computer in which the hard disk or the like is assembled.
As a technique for the elimination of such an inconvenience, Japanese Patent Publication (Kokoku) No. SHO 60-48668 proposes, as shown in FIG. 19, to form oil-repelling films 12,12 of polytetrafluoroethylene (PTFE) in the vicinity of a gap 11 in which the magnetic fluid 10 is retained, so that the magnetic fluid 10 is prevented from spreading out of the gap 11.
The above patent publication, however, discloses nothing more than the formation of such oil-repelling films. It has therefore been difficult to construct a magnetic fluid sealing device having sufficient practical utility.
To provide at a low cost a magnetic fluid sealing device having good performance, high durability and high reliability, it is necessary to make it possible to efficiently form a film having good peeling resistance and oil-repelling property while making its thickness as thin as possible. None of these conditions, however, can be fully satisfied by the formation of the oil-repelling films 12,12 with PTFE or the like as disclosed in the above patent publication.
When the oil-repelling films 12,12 made of PTFE are formed, for example, as disclosed in the above patent publication, an additional cost is required for the formation of the films and, moreover, the oil-repelling films 12,12 so formed have a large thickness. The dimension of the gap 11 in which the magnetic fluid 10 is retained tends to vary, leading to the potential problem that the magnetic fluid 10 cannot be retained in a desired amount. If the oil-repelling films 12,12 made of PTFE are peeled off, relatively large dust may float in a clean space in which a hard disk or the like is accommodated and may also become a cause for a malfunction of a computer with the hard disk or the like assembled therein.
If the film surface is rough or contains scratches or the like, the magnetic fluid tends to spread along recesses or scratches. Spreading of the magnetic fluid tends to contaminate the clean space due to the scattered magnetic fluid.
With a view to overcoming such an inconvenience, the present inventors have already invented magnetic fluid sealing devices in each of which an oil-repelling layer in the form of an adsorbed monomolecular layer is formed on an intermediate surface area between a fluid-deposited surface area and a fluid-free surface area by causing a fluorinated surfactant to be adsorbed on the intermediate surface area (copending U.S. patent application Ser. No. 07/527,909 filed May 25, 1990). In the magnetic fluid sealing devices according to such previous inventions, an extremely-thin oil-repelling layer having extremely high peeling resistance can be formed. Under severe use conditions, there are however still accompanied by such unsolved problems as will be described next.
Preferably, molecules of a fluorinated surfactant forming an adsorbed monomolecular layer should be arranged in good order without intervals on the intermediate surface area as depicted in FIG. 20. Molecules of the fluorinated surfactant are however adsorbed at random on the surface area as shown in FIG. 21 because they do not have two-dimensional bonding force, i.e., bonding force in the direction of a plane in which the monomolecular layer lies.
Nevertheless, the adsorbed monomolecular layer has sufficient practical utility as long as it is used under normal use conditions. Under severe use conditions, it may however be found insufficient in durability, weatherability and the like.